It has been known for many years that amorphous alloys of metals and non-metals combinations (particularly transition metal-phosphorus alloys such as nickel phosphorus alloys) have many desirable properties, including excellent corrosion resistance, controllable magnetic and electrical responses, good wear properties, and unusual mechanical behavior
Present methods for producing amorphous nickel phosphorus alloys, or like alloys of metal and non-metal combinations, each have significant drawbacks. For instance, the technique of splat cooling in which the alloy is rapidly solidified from the melt offers only limited applications, as it has constraints which limit the geometries of the end products to ribbons or sheets. While splat cooling can achieve a product of desirable ductility, the splat cooled coating has a maximum achievable thickness of 1 mil.
Vacuum deposition methods also are known, but they are inherently rate limited to thin coatings in reasonable time periods. Plasma arc deposition produces non-compact coatings with low densities. Electroless deposition produces brittle coatings at rates rarely in excess of 0.0008 inches per hour, often unacceptable for commercial processes.
Electrodeposited alloys of transition metals and phosphorus, such as alloys of nickel and/or cobalt and phosphorus, have reasonably good deposition rates (0.001 inch-0.005 inch per hour), result in a product that is superior from a corrosion resistance standpoint, and have a wide variety of other advantages. However, typical electrodeposition techniques produce alloys having limited ductility (e.g. about 1 percent elongation). This limited ductility prevents forming operations after coating, and results in limitations on deposition rates utilizing standard operating conditions in the electroplating industry.
According to the present invention, the advantages of electrodeposition of transition metal-phosphorus alloys can be maintained while at the same time producing alloys having sufficiently good ductility properties so that the alloys may be used on many products where their use is presently precluded. Examples of such areas of use include magnetic recording tape and textile printing screens. The alloy can be used in making orifice plates according to the teachings of U.S. Pat. No. 4,528,070 commonly assigned herewith. Orifice plates can be made by plating the transition metal-phosphorus alloy (typically nickel and/or cobalt phosphorus alloy) on a substrate such as stainless steel, and then stripping the transition metal-phosphorus alloy off the stainless steel so that the alloy has a foil configuration that serves as its own orifice plate and support. These are merely a few examples of a wide variety of uses to which alloy film configurations according to the invention can be put, either as coatings on substrates, or as unsupported foils.
The nickel phosphorus alloys according to the present invention have greatly enhanced ductility properties, whether measured qualitatively or quantitatively. For example, as a representation of the excellent ductility properties which may be demonstrated qualitatively, an unsupported amorphous nickel phosphorus alloy foil can be produced according to the invention having a thickness of greater than 1 mil (i.e. greater than can be obtained by splat cooling) and having ductility properties such that it may be formed into a complex geometric shape, such as twisted into a helix or accordion folded, without cracking. In addition, the alloy according to the invention is fully specular in appearance when plated to any thickness (i.e. it is highly reflective without distortion), and it maintains the structure and integrity of the underlying surface as prepared for coating, without degradation of the surface smoothness. The alloy can be deposited at conventional electrodeposition rates, i.e. at least about 0.001 inch per hour, and has been applied at rates up to and above about 0.020 inch per hour.
Measured quantitatively, if a film configuration of alloy according to the present invention is in foil form, its ductility is comparable to at least about 5 percent (and can be greater than about 10 percent) for a 25 micron foil subjected to the ASTM Standard Practice for Micrometer Bend Test for Ductility of Electrodeposits (ASTM designation B490-68 as reapproved 1980).
The preferred alloy according to the present invention is produced in an electroplating bath which typically comprises about 0.5-1.0 molar nickel chloride, about 1.5-3.0 molar phosphorous acid, about 0.1-0.6 molar phosphoric acid, and about 0.0-0.6 molar hydrochloric acid. The bath must have at least 1.25M Cl.sup.-, and there must be at least twice the amount of Cl.sup.- in the bath as Ni.sup.+2. While the exact mechanism that results in the desired end product according to the invention is not completely understood, it is believed that the enhanced ductility achieved according to the invention is due to lower amounts of codeposited hydrogen in the electrodeposit, brought about by the presence of hydrochloric acid, and an excess of chloride ions with respect to nickel ions in the bath. However, if the alloy is to remain resistant to nitric acid corrosion, the upper limit of the chloride in the bath is about 2.0 molar.
It is the primary object of the present invention to produce transition metal phosphorus alloy film configurations, and particularly nickel phosphorus coatings or unsupported foils, having excellent ductility, while retaining good corrosion resistance typical of such alloys. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.